In semiconductor manufacturing processing, layers of material on wafers are built up and selectively removed. For example, small portions of layers may be removed using a selective etching process, e.g., using a photolithographic process. Other times, the wafers are processed to planarize the surface of the wafer. Without uniform surfaces, an accurate circuit pattern cannot be obtained from the next photolithographic process. In order to planarize the surface of a wafer, chemical mechanical polishing (CMP) is often used. During CMP, the wafer is spun or rotated while it is pushed toward a polishing pad, which is sprayed with a slurry, i.e. a suspension which includes inorganic particles and surfactants. The wafer surface is polished flat through the combination of mechanical friction between the wafer and suspension and chemical dissolution reaction of the suspension.
Accurate measurement of the layers on the wafer during or after processing is important to ensure the proper amount of polishing or etching of the materials. By way of example, end point detection is used with CMP processing to determine when the desired amount of material has been removed. Spectroscopic reflectometry or other types of non-destructive optical metrology is often used to determine the thicknesses of the layers after processing. To determine the thickness of layers (or other desired parameters of the layers), conventional modeling techniques are often used, in which the thicknesses of the top layer and the underlying layers are varied until an acceptable match between the modeled data and the measured data is achieved. Unfortunately, the thicknesses of some materials, such as oxide and nitride, exhibit strong correlation particularly near the end-point. Accurate and precise measurement of strongly correlated thicknesses using conventional methods is difficult.
Accordingly, what is needed is an improved metrology method, e.g., which can measure strongly correlated thicknesses or other parameters.